On-chip integrated circuit (IC) capacitors commonly employ metal plates separated by a thin deposited or thermally grown dielectric layer. Polycrystalline silicon (polysilicon) plates have proven to be useful particularly where thermally grown oxide dielectric layers are employed. In addition, plates composed of silicon-metal compounds called silicides have proven to be useful as capacitor plates. Such capacitors can be fabricated using conventional integrated circuit fabrication processes and they can be made to close tolerances. However, such capacitors commonly have an unacceptably high voltage coefficient of capacitance. Such a coefficient means that the value of the capacitor varies with applied voltage.
In many applications the large coefficient has no significant effect and can be ignored. However, when the capacitor is to be used in a D/A or A/D converter and serves as a charge distribution element, high accuracy requires a low voltage coefficient of capacitance. A 16-bit converter requires a coefficient of less than .+-.15 ppm over a .+-.10 volt range. A typical IC thin film capacitor will have a coefficient of almost .+-.500 ppm and is therefore, quite unsuitable.
U.S. Pat. No. 4,628,405 by John A. Lippert issued Dec. 9, 1986, to the assignee of the present invention. This patent discloses improvements in thin film capacitor fabrication to produce voltage coefficients in the few ppm range. The teaching in this patent is incorporated herein by reference. The essential teaching in the patent is that the large value of voltage coefficient is the result of trapping states in the oxide dielectric. Means are disclosed for greatly reducing trapping states. One effective means is the use of refractory metal or refractory metal silicide capacitor plates and the avoidance of surface contamination prior to dielectric formation. Using the teaching in this patent can produce a capacitor having a very low voltage coefficient. However, special processing must be employed. It would be helpful to employ more conventional processes to produce capacitors having precise capacitance values and an acceptably low voltage coefficient of capacitance.